1. Technical Field
The present invention relates to a radiographic imaging system, and in particular, to a radiographic imaging system in which fluoroscopic imaging, that carries out capturing of radiographic images continuously, is possible.
2. Related Art
Radiation detectors such as FPDs (Flat Panel Detectors), in which a radiation-sensitive layer is disposed on a TFT (Thin Film Transistor) active matrix substrate and that can convert radiation directly into digital data, and the like have been put into practice in recent years. Portable radiographic imaging devices (hereinafter also called “electronic cassettes”), that capture radiographic images expressed by irradiated radiation by using the radiation detector, are being put into practice. As compared with a radiographic imaging device that uses a conventional X-ray film or imaging plate, a radiographic imaging device that uses the radiation detector has the advantages that images can be confirmed immediately, and fluoroscopic imaging (video imaging), in which the capturing of radiographic images is carried out continuously, also can be carried out. Note that, as methods of converting radiation at the radiation detector, there are an indirect conversion method that, after converting radiation into light at a scintillator, converts the light into charges at a semiconductor layer of photodiodes or the like, and a direct conversion method that converts radiation into charges at a semiconductor layer of amorphous selenium or the like, and the like. There exist various materials that can be used at the semiconductor layer in these respective methods.
As imaging methods for fluoroscopic imaging, there are a method of capturing images at a predetermined frame rate while irradiating radiation continuously from a radiation source (continuous irradiation), and a method of, while irradiating radiation in the form of pulses synchronously with the frame rate (pulse irradiation), capturing images synchronously with the irradiation of the radiation. With pulse irradiation, radiation can be irradiated for the time period needed for imaging, and the exposure dosage of the patient can be suppressed as compared with continuous irradiation, and there is therefore the advantage that the irradiated amount per unit time is increased. However, with pulse irradiation, there is the need to synchronize the timing of the image capturing at the radiation detector with the timing of irradiating the radiation from the radiation source.
Japanese Patent Application Laid-Open (JP-A) No. 2009-17476 discloses a technique of preventing deterioration in the S/N of a radiographic image and avoiding narrowing of the dynamic range, even in a case in which the frame rate is lowered as needed and the exposure dosage of the subject is reduced. In accordance with this technique, on the basis of the frame time of one frame, a first accumulation time, that relates to accumulation of charges that is carried out at the time of obtaining a radiographic image, is computed. If the first accumulation time is greater than a reference time that is a reference, a dummy accumulation time of charges is set within the frame time of one frame, and the first accumulation time is changed to a second accumulation time that is smaller than this first accumulation time. Then, image capturing processing of a radiographic image is carried out on the basis of the second accumulation time.
JP-A No. 6-304162 discloses a technique in which an incident X-ray amount detecting means is provided that detects an incident X-ray amount that has passed through a subject and is incident on a film, in a case in which pulse X-rays of a predetermined pulse interval that is in accordance with a predetermined tube voltage and tube current are generated from an X-ray tube and a tomogram of a cut section of the subject is captured on the film. The incident X-ray amount on the film, that is detected by the incident X-ray amount detecting means during the tomographic imaging, is integrated. When the integral value that is integrated per one pulse exceeds a threshold value of an incident X-ray amount that should be incident on the film, exposure of X-rays is cut-off.
In pulse irradiation, because the irradiation time period is short, the respective images may become frame-advanced images having stopped motion. This trend occurs in particular in cases in which the frame rate is low, because the imaging interval is large and after-images of the eyes also disappear.